This paper mainly deals with vibration problems in a marine COGAG plant and the remedies adopted. In addition to the commonly-found vibration caused by an imbalance of the rotor and resonance in the support structure, there was also random vibration in a narrow low-frequency band caused by a disturbance in the exhaust flow. These sources of vibration were identified and their amplitude successfully decreased from 0.16mm to 0.018mm by appropriate countermeasures.